In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A class of imageable elements called printing plate precursors, useful for making lithographic printing plates, comprises a photosensitive layer over the hydrophilic surface of a substrate. The photosensitive layer comprises one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can be the binder material itself.
If after exposure to radiation, the exposed regions of the photosensitive layer are removed in the developing process, revealing the underlying hydrophilic surface of the substrate, the element is referred to as positive working. Conversely, if the unexposed regions are removed by the developing process, the element is negative working. In each instance, the regions of the radiation-sensitive layer that remain (i.e., the image areas) are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution, and repel ink.
Direct digital imaging of offset printing plates, which obviates the need for exposure through a negative, is becoming increasingly important in the printing industry. High-performance lasers or laser diodes, which are typically used to image these plates, emit radiation between 800 and 1100 nm. Therefore, printing plate precursors that are to be imaged by these radiation sources must be sensitive to radiation in this wavelength region. Such printing plate precursors may be handled in ambient light, which significantly facilitates their production, handling and processing.
Negative working lithographic printing plate precursors which can be imagewise exposed with infrared lasers are described for example in EP-A-0 672 544; EP-A-0 672 954; DeBoer, U.S. Pat. No. 5,491,046; and EP-A-0 819 985. However, the usefulness of these printing plate precursors is restricted by their shelf life, when stored in a hot and/or humid atmosphere. This shelf life issue makes plates usable in some cases for only one month or less. Thus, a need exists for negative working printing plate precursors with a longer shelf live.